Instrument for measuring the topography of a surface

ABSTRACT

An instrument for measuring the topography of a surface, comprising a light transmitting device for transmitting a light beam onto the surface; a light receiving device for receiving light reflected by the surface; a detector for detecting the position of the received light relative to the optical axis of the light receiving device; and means for providing relative movement between on one hand the light transmitting (10,L1,L2) and light receiving (L3,L4,8) devices, and on the other hand the surface (2) to be measured. According to the invention the optical axis (11) of the light transmitting device (10,L1,L2) is arranged to form a small angle (γ 1 ) with the normal to the median plane (13) of a workpiece surface (2) to be measured. The angle (γ 1 ) is smaller than about 15°. The optical axis (9) of the light receiving device (L3,L4) forms a right angle, or substantially a right angle (8) with the optical axis of the light transmitting device. The detector (8) is position-responsive, and an image magnifying lens (14) is located in front of the detector.

The present invention relates to an instrument for measuring thetopography of a surface. By the topography of a surface is meant boththe micro and the macro irregularities in the surface.

The present invention utilizes the known principle of transmitting abeam of light onto the surface being measured and receiving the lightreflected by the surface and therewith determine the topography orirregularities of the surface.

Such instruments are known to the art. These known instruments, however,are encumbered with several drawbacks, one of which is that the incidentlight impinges on the surface at a significant angle, for example 45°,and/or that the light reflected from the surface is received in adirectional sense having a corresponding angle to the surface.

When the incident light impinges on the surface at a significant anglethereto, an error is obtained in the positional reading in the plane ofthe surface, hereinafter referred to as the horizontal positionalreading. When traversing a totally flat surface in a directionperpendicular to the surface, the light spot produced on the surface bythe incident beam will be displaced across the surface. Correspondingly,the light spot is displaced relative to the surface as a result of thedepth profile of the surface. Thus, when the depth profile of a surfaceis measured with an instrument with which the incident light forms asignificant angle with the surface and the surface is traversed in thehorizontal plane, it is not known exactly where a given profile depthprevails, and hence an error is created in the horizontal positionalreading, as before mentioned.

Another serious disadvantage is that when the reflected light isreceived by an optical system in which the optical axis thereof forms asignificant angle with the surface, e.g. 45°, the vertical dissolutionwill be relatively poor, owing to the fact that the projected surface ofthe light spot becomes large in relation to the profile depth. Thisgreatly influences the total resolving power of the instrument.

Another drawback experienced when the incident light beam is permittedto impinge on the surface at a significant angle thereto, e.g. 45°, isthat the mean size of the light spot over a surface to be measuredbecomes greater than the diameter of the incident light beam.

Present day instruments of this kind are intended to measure relativelyflat surfaces, where the profile depth is of the order of micrometres.In this measurement range the aforedescribed drawbacks become highlysignificant.

These drawbacks are eliminated, or at least greatly alleviated, by meansof the present invention, which provides an instrument with which veryhigh resolution can be achieved.

Thus, the present invention relates to an instrument for measuring thetopography of a surface, comprising a light transmitting device fortransmitting a light beam onto the surface; a light receiving device forreceiving light reflected by the surface; a detector for detecting theposition of the received light relative to the optical axis of the lightreceiving device; and means for providing relative movement between onone hand the light transmitting and light receiving devices, and on theother hand the surface to be measured, the instrument beingcharacterized in that the optical axis of the light transmitting deviceis arranged to form a small angle with the normal to the median plane ofa workpiece surface to be measured, said angle being smaller than about15°, and preferably smaller than about 5°; in that the optical axis ofthe light receiving device forms a right angle, or substantially a rightangle, with the optical axis of the light transmitting device; in thatsaid detector is position-responsive; and in that an image magnifyinglens is located in front of the detector.

The invention will now be described in more detail with reference to anexemplifying embodiment thereof illustrated in the accompanying drawing,in which

FIG. 1 illustrates schematically an instrument according to theinvention; and

FIG. 2 is a block schematic of an electronic part of the instrument.

FIG. 1 illustrates schematically an instrument 1 according to thepresent invention. The instrument 1 is intended for measuring thetopography of a surface 2. The surface 2 is the surface of a workpiece 3carried by a schematically illustrated table 4. The table 4 is arrangedfor movement in a direction shown by the arrow 5 and/or in a directionperpendicular to the plane of the paper.

The instrument 1 includes a light transmitting device for transmitting alight beam 6 onto the surface 2, and a light receiving device forreceiving light 7 reflected from the surface, and further includes adetector 8 for detecting the position of the received light relative tothe optical axis 9 of the light receiving device.

Both the light transmitting device and the light receiving device areincorporated in a housing 17 suspended from an axle 18 or the like.

The light transmitting device may consist of a low power laser, forexample an He-Ne-laser 10, which transmits a laser beam which isfocussed on a distance where the surlaser face 2 is to be found, bymeans of a conventional lens system comprising, for example, two lensesL1 and L2.

According to the invention the optical axis 11 of the light transmittingdevice is arranged to form a small angle γ₁ with a normal 12 to the mainplane extension 13 of the workpiece surface 2. The angle γ₁ shall besmaller than about 15°, and preferably smaller than about 5°. The anglemay thus also be 0° (zero degrees) This means that the aforesaidhorizontal positional reading can be effected with a high degree ofaccuracy, and that the resolution of the instrument is much greater thanif the angle γ₁ were larger, e.g. 45°.

According to the invention, the optical axis 9 of the light receivingdevice forms a right angle, or substantially a right angle δ with theoptical axis 11 of the light transmitting device. The angle δ shallexceed about 75°, and preferably exceeds 85°.

This affords the advantage that the light spot illuminated on thesurface 2 will be held constantly in the focus of the imaging lenssystem. Because the angle γ₁, and therewith the angle γ₂, between theplane 13 and the optical axis 9 of the imaging lens system are small andequal, or substantially equal, it is not possible for reasons of spaceto place a highly magnifying lens system close to the surface 2 alongthe optical axis 9 of the imaging lens system. Consequently, there isplaced in the beam path of the light receiving device a relay lens L3which captures the light reflected against the surface 2. In accordancewith one embodiment of the invention, a mirror S1 is arranged to reflectlight arriving through the relay lens. The relay lens L3 has a longfocal length, e.g. 30 mm.

According to the invention the light receiving device includes an imagemagnifying lens L4. The image magnifying lens has a so-called microscopelens of high magnification. For example, the focal length is only 2.5mm.

In accordance with one preferred embodiment there is located downstreamof the microscope lens L4, in the beam path, a semi-transparent mirrorS2, which is arranged to conduct light to the detector 8 and to anocular or eyepiece 14, through which the light spot can be observed onthe surface 2 with the eye, the light spot being used to adjust thesetting of the optical axes 11, 9, so that the light spot can be imagedwith full sharpness on the surface 2.

The sharpness of the image is adjusted by displacing the housing 17towards or away from the surface 2, with the aid of known suitablesetting means, such as gear racks which motivate the aforesaid axle 18.

The use of a microscope lens is an extremely important feature, sincethe magnification obtained therewith in combination with aposition-responsive detector 8 whose size is consequently adaptedthereto results in high resolution of the instrument. Theposition-responsive detector is of a known kind readily available on themarket and is arranged to produce an electric outputsignal in responseto the position where the light spot strikes the detector surface 15, inthe form of a coordinate. The detector 15 is also of the kind with whichthe aforesaid coordinate is given as the centre of mass of the lightspot impinging on the detector.

Because the laser light has Gaussian intensity distribution, thedetector will give a highly accurate indication of the position of thelight spot on the detector. Any suitable detector may be used. Forexample, the detector used may be one designated type LSC4 and retailedby United Detector Technology, Hawthorn, Calif., USA.

Thus, a laser beam 6 is focussed on the surface 2 whose surfacesmoothness or surface configuration is to be measured. This is effectedwith the lens system L1, L2, with the aid of which it is also possibleto expand the beam and to cause the beam to impinge on the surface atthe incidence angle γ₁. An illuminated spot is obtained on the surfacein this way. This spot is imaged with the lens system L3, L4, via themirror S1, on an image plane B2, in which the position-responsivedetector 8 has been placed.

When the illuminated spot on the surface is located precisely along theoptical axis 9 of the imaged lens system, the image of the illuminatedspot is obtained at location A on the detector 8. When the illuminatedspot is located beneath the optical axis 9 of the imaging lens system,the image is obtained at location C on the detector, while when theilluminated spot is located above said optical axis the image isobtained in location B.

By translating either the workpiece surface 2 or the instrument housing17 in horizontal direction referenced 5, the surface 2 is caused to beilluminated at a number of points along a line. The illuminated spotwill herewith move up and down in accordance with the topography of thesurface and in dependence on whether the surface is locally above orbeneath the optical axis 9 of the imaging system. The position detectedis a direct measurement of the local distance between the surface and areference plane, which may comprise the median plane 13 of the surface2.

Resolution of the instrument in the vertical direction 16 is determinedby the imaging lens system. Consequently, it is important that the angleγ₁ in FIG. 1 is as small as possible, otherwise there is obtained asignificant error in the horizontal position reading, as discussed inthe aforegoing. In addition, the optical axis 11 of the lighttransmitting device and the optical axis of the imaging optical systemshall form a right angle, or substantially a right angle, sinceotherwise the illuminated spot will not be sharply imaged or reproducedwhen the profile depth varies, as before mentioned.

The relay lens L3 gives a primary image of the illuminated spot in animage plane B1. This image should be obtained at a distance from theoptical axis 7 extending between the mirror S1 and the lens L4 which isof the same order of magnitude as the distance between the illuminatedspot and the optical axis 9 of the receiving lens system. This distanceis too small to be measured accurately with a position-responsivedetector. Consequently, as before mentioned, the image B1 is reproducedon the image plane B2 with the aid of the microscope lens L4, therewithgreatly magnifying the distance between the image of the illuminatedspot and the optical axis 9. It is possible when using the describedinstrument to (a) utilize a small angle γ₁ and therewith obtain accuraterepresentation of the horizontal position, (b) to arrange that γ₂ =γ₁and therewith ensure that the illuminated spot is always in the focus ofthe imaging lens system, and c) to use a magnifying lens L4 of highmagnification, so that small profile depths can also be measuredaccurately.

The use, in principle, of an ocular or eye-piece for manual viewing, asillustrated in FIG. 1, enables the instrument to be adjusted or set tothe correct distance from the surface whose structure is to bedetermined. This is done by viewing the surface through the ocular 14.When the illuminated part of the surface 2 is sharply imaged, it meansthat this part of the surface is in the focus of the imaging lenssystem.

The resolving power of the instrument can be calculated as horizontalresolution, i.e. the size of the light spot on the surface 2, and asvertical resolution, i.e. the vertical position of the light spot inrelation to the reference plane.

The following calculations of the possible resolving power of theinstrument are based on the assurtion that the light transmitted is alaser. A resolving power of the same order of magnitude can be obtainedwith a convention light source, however.

The size d of the illuminated spot (and therewith the spatial resolvingpower of the instrument) is a function of the diameter (D) of theincident beam at the exit of the lens L2, the focal length f₂ of thelens L2, and the distance of the surface 2 from the theoretical focus ofthe beam. The following equation then applies: ##EQU1## where λ is thewavelength of the laser light, F=f₂ /D, and z is the distance betweenthe lens L2 and the illuminated spot. It is possible, by varying F, toobtain varying resolution and maximum profile depth with retained focuson the surface 2.

A laser beam which passes through a lens system will contract downstreamof the lenses, so that the beam of light rays obtains a waist. Bymaximum profile depth with retained focus is meant that the length ofthe beam in the region where the beam diameter along its waist issmaller than a given diameter (d). In the following Table, Table 1, theaforesaid length is shown in dependence on the maximum diameter (d) formaintaining the requisite focus in dependence on the magnitude F. Thus,focussing on the surface is maintained along the aforesaid waist.

                  TABLE 1                                                         ______________________________________                                        Maximum profile depth with retained focus in                                  mm for λ = 0.6328 μm                                                diameter (d) of                                                               the beam in μm                                                                         F = 10  F = 20    F = 50                                                                              F = 100                                   ______________________________________                                         5          0.118                                                             10          0.199                                                             20          0.400   0.474                                                     50          1.000   1.893     2.962                                           100         2.000   3.948     9.153 11.846                                    200         4.000   7.974     19.590                                                                              36.611                                    ______________________________________                                    

It will be seen from Table 1 that when using a light spot having adiameter of 20 μm, there is obtained a maximum permitted variation inprofile depth of 0.4 mm for an F-10 lens, and a somewhat larger profiledepth of 0.474 mm when an F-20 lens is used. If a higher degree ofresolution is required, a lens of smaller F-number than 20 must be used,wherewith the permitted profile depth decreases. If a large light spotcan be allowed, however, a lens of higher F-number can be used, with asubsequent increase in the permitted profile depth.

It should be mentioned here that the diameter of the light spot is aconservative measurement of the horizontal resolving power of theinstrument. The position-responsive detector⁸ produces an output signalwhich is proportional to the centre of mass of the light spot imaged onthe detector. This means that the "effective" diameter of the light spotis only a fraction of the total diameter.

The choice of lens used and the size of light spot applied can beadapted from case to case by those skilled in this particular art.

With regard to the resolution of the instrument in the verticaldirection., i.e. the vertical position of the illuminated spot inrelation to a reference plane, which may be the plane 13, it isnecessary to carry out a comprehensive investigation in order toestablish this resolution. Consequently, all that is given here is anappreciation of the resolution capable of being obtained under the worstconceivable circumstances, and an appreciation of typical workshopconditions.

When viewed through the imaging lens system, the illuminated spot isseen as an ellipse-like surface. Its width is equal to the diameter (d)of the beam, while its height is the width projected on a planeperpendicular to the optical axis (9) of the imaging lens system. Theheight can be expressed as ##EQU2## where h is the projected height, andα is the angle formed locally by the surface with the reference plane13, vide FIG. 1. It was previously stated that the angle γ₂ shall beequal to γ₁, and that these angles shall be small. In practice the angleα is also very small. In the case of those surfaces of interest from theaspect of workshop practice and technology, the angle α may reach 5degrees. In this case the following approximative equation applies:

    δh=d·(γ.sub.2 -α)

where the angles are expressed in radians. The height of the imaged"ellipse" in relation to its width is given in Table 2 below.

                  TABLE 2                                                         ______________________________________                                         ##STR1##                                                                     α = -5°                                                                       α = -2°                                                                   α = 0°                                                                    α = +2°                                                                 α = +5°                    ______________________________________                                        γ.sub.2 = 5°                                                             0.175    0.122    0.087  0.052  0.000                                 γ.sub.2 = 7°                                                             0.202    0.157    0.122  0.087  0.035                                  γ.sub.2 = 10°                                                           0.262    0.209    0.175  0.140  0.087                                 ______________________________________                                    

It will be seen from Table 2 that in the worst case the height of theprojected illuminated light spot can constitute a quarter of its widthin respect of the angles given in the Table.

It must be emphasized once more that the position-responsive detectorweights the output signal against the centre of mass of the light spot.This means that the resolving power of the instrument in the verticaldirection will be much greater than that given in Table 2. Theimprovement achieved thereby is restricted mainly by the local curvatureof the surface. In the extreme case when the surface is totally flatlocally, the centre of mass is exactly in the centre of the lightsource. In conditions such as these the resolution is determined solelyby the degree of magnification selected and the resolution in the actualdetector itself. The degree of magnification is selected so that thetotal profile depth is imaged on the whole length of the detector. Inthis way, with the aid of the aforesaid commercial detector 8, theinstrument can be given a total resolving power according to Table 3below.

                  TABLE 3                                                         ______________________________________                                        The total resolving power of the                                              instrument in the vertical direction (μm)                                  d μm  F = 10  F = 20     F = 50                                                                              F = 100                                     ______________________________________                                         5       0.0054                                                               10       0.0100                                                               20       0.020   0.024                                                        50       0.05    0.095      0.148                                             100      0.100   0.197      0.458 0.593                                       200      0.200   0.399      0.980 1.830                                       ______________________________________                                    

It will be seen herefrom that a particularly high resolving power isobtained.

The drawbacks referred to in the introduction and associated with knowninstruments are therewith eliminated, or at least substantially reduced.

It has been said in the aforegoing that the angle γ₁ is preferablysmaller than 5°. In addition the angle must be adapted to the surface ofthe workpiece under examination. It should be mentioned, however, thatthis angle γ₁ may be 0° (zero degrees). The only essential requirementis that a light spot on the surface can be seen by at least the upperpart of the lens 3 in FIG. 3 in order to be focussed on the detector 8.

It will have been noticed that measurements are made with the instrumentaccording to the present invention in a contactless fashion. Thisprevents possible deformation of the surface 2 during a measuringprocess. Consequently, the instrument can be used to measure all mannerof materials, such as metals, ceramics, rubbers, leather, paper, etc.

It has been said in the aforegoing that the table 4 can be displaced inone or two directions, suitably in a horizontal plane, and that theinstrument housing 17 can be moved towards and away from the table 4, inorder to adjust the sharpness of the transmitted light beam on thesurface 2.

It will be understood, however, that the table 4 may, alternatively, bestationarily arranged and the housing 17 arranged (a) for movement in adirection or in a plane parallel with the table 4, for scanning orsensing a surface 2, and (b) for movement towards and away from thesurface 2 for bringing the illuminated spot to the requisite degree ofsharpness. In this case the axle or shaft 24 is connected to mechanicaldevices (not shown) for example of the aforementioned kind, arranged toeffect the aforesaid movements.

FIG. 2 is a schematic illustration of a block schematic of an electronicpart of the instrument. As before mentioned, the detector 8 produces anelectric signal corresponding to the position of the light spot on thedetector. This signal is sent to a microprocessor 18 or the like.Position sensors or detectors 19,20 are also arranged to send to thedataprocessor 18 a signal relating to the position of the table 4 inrelation to the housing 17. In one embodiment the position detectors arearranged to give said position in an x-y-plane coinciding with thehorizontal plane in which the table can be moved. In another embodimentthe position detectors 19,20 can be arranged to give the position of thehousing 17 in a corresponding x-y-plane in that case when the housing ismovable and the table 4 stationary.

The microprocessor 18 is arranged, in a known manner, to process thesignal arriving from the detector 8 with the signals from the positiondetectors 19,20, therewith to enable a curve showing the topography ofthe surface or the measurement values thereof with regard to the profiledepth of said surface at given positions or along given lines to beillustrated or disclosed.

Thus, according to the described embodiments, the position of the lightspot on the detector varies with the topography of the surface.

According to another embodiment of the invention, the distance betweenthe housing 17 and the surface 2 is controlled so as to be constant thewhole time. In this case the microprocessor 18 is arranged to controlpositioning means 21,22 incorporating electric motors which drive theaforesaid housing setting devices. In this respect the microprocessor 18is arranged to control the positioning means 21,22 so that the point atwhich the light spot impinges on the detector is constant, preferably inthe centre point A of the detector. In this case the table 4 can eitherbe movable and the housing 17 stationary, or vice versa, the positiondetectors or sensors 19,20 being connected respectively to either thetable 4 or to the housing 17. According to this embodiment there isprovided a further position detector or sensor 23 which is arranged tomeasure the vertical distance between the housing and the table.

Thus, in this embodiment, the housing moves up and down in conformitywith the topography of the surface 2. In this respect the last mentionedposition indicator 23 is arranged to produce an electric signal whichcorresponds to the topography of said surface. This signal issignal-processed in the microprocessor in a manner corresponding to thataforedescribed with regard to the signal from the detector.

The advantage with this embodiment is that a very high degree ofresolution is obtained in both the horizontal and vertical directions,while permitting at the same time a very wide variation in profiledepth. The embodiment affords a particular advantage when the surfacehas a pronounced curvature.

Thus, with this embodiment it is possible to use the present instrumentas a distance meter and therewith measure, for example, the roundness,conicity, etc. of an element. With regard to these latter applicationsthe workpiece to be measured, for example, may be mounted on an axlewhose rotational position is given by a position indicator 19 while itsaxial position is given by the other position indicator 20. In thiscase, the position of the housing relative to the surface indicated bythe position indicator 23 constitutes a measurement of the roundness orconicity of the element.

The light transmitting device may alternatively comprise a conventionallight source, the light emanating from which is focussed on a smallaperture of from 5 to 50 μm in size. The light passing through theaperture is focussed on the point where the surface 2 is to be found,e.g. with the aid of two lenses L1 and L2.

The lenses L1 and L2 may be exchangeable with other lenses of otherfocal length and/or may be displaceable in relation to one another andto the light source, or may be replaced with a zoom-lens system. Any oneof these alternatives will enable variations to be made in (a) thethickness of the exiting light beam, (b) the angle γ₁ formed between theoptical angle 11 of the beam 6, and (c) the normal 12 to the mainextension plane 13 of the workpiece 3.

It will be obvious that many variations are possible, particularly withregard to the application in question.

Consequently, the present invention shall not be considered to berestricted to the described and illustrated embodiments, sincemodifications and variants can be made within the scope of the followingclaims.

I claim:
 1. An instrument for measuring the topography of a surface,comprising a light transmitting device for transmitting a light beamonto the surface; a light receiving device for receiving light reflectedby the surface; a detector for detecting the position of the receivedlight relative to the optical axis of the light reeiving device; andmeans for producing relative movement between on one hand the lighttransmitting (10,L1,L2) and light receiving devices (L3,L4,8) and on theother hand the surface (2) to be measured, characterized in that theoptical axis (11) of the light transmitting device is arranged to form asmall angle (γ₁) with the normal of the median plane (13) of a workpiecesurface (2) to be measured, said angle (γ₁) being smaller than about15°in that the optical axis (9) of the light receiving device (L3,L4)forms substantially a right angle (δ) with the optical axis (11) of thelight transmitting device; in that said detector (8) ispositionresponsive; and in that an image magnifying lens (L4) is locatedin front of the detector (8), said image magnifying lens (L4) being amicroscope lens of short focal length.
 2. An instrument according toclaim 1, characterized in that a relay lens (L3) is placed in the beampath of the light receiving device and arranged to capture lightreflected by said surface (2); and in that there is provided a mirror(S1) for reflecting light entering through the relay lens (L3) to themicroscope lens (L4).
 3. An instrument according to claim 2,characterized in that there is arranged in the beam path of the lightreceiving device downstream of the microscope lens (L4) asemi-transparent mirror (S2) for conducting light to the detector (8)and to an ocular (14).
 4. An instrument according to claim 2,characterized in that the light transmitting device includes a laser(10) with an associated lens system (L1,L2).
 5. An instrument accordingto claim 2, characterized in that the light transmitting device includesa zoom lens system.
 6. An instrument according to claim 1, characterizedin that the means for producing said relative movement are arranged,during said relative movement, to maintain a constant distance betweenon one hand the light transmitting (10,L1,L2) and light receivingdevices (L3,L4,8) and on the other a reference plane (13) of theworkpiece (3), the point at which the light received via the lightreceiving device (L3,L4) impinging on the detector varying with thetopography of the surface (2) of the workpiece (3).
 7. An instrumentaccording to claim 1, characterized in that the means for providing saidrelative movement are arranged to displace a reference plane (13) of theworkpiece (3) relative to the light transmitting (10,L1,L2) and lightreceiving (L3,L4,8) devices in the inherent plane of the reference plane(13), and to displace the light transmitting and light receiving devicesin direction (16) towards and away from said direction (16) towards andaway from said surface (2) so that the impingement point of the lightreceived, via the light receiving device, on the detector (8) isconstant; and in that a position sensor (23) is provided for detectingvariations in distance between on one hand the light transmitting andlight receiving devices and the reference plane (13) on the other, thesevariations corresponding to the topography of the workpiece surface (2).8. An instrument as defined in claim 1, wherein said angle (γ₁) issmaller than approximately 5 degrees.
 9. An instrument as defined inclaim 1, wherein said light transmitting device includes means whichtransmits a light beam onto the surface and illuminates a sharp andsmall spot on the surface.